How did Xerxes I gather troops and ships from across the Achaemenid Empire?

Xerxes I gathered troops and ships for his invasion of Greece by leveraging the established administrative and military structure of the Achaemenid Empire. Under this system, the empire was divided into approximately 20 provinces known as satrapies, each governed by a satrap who held full civil and military authority.1

The conquered peoples within these territories were obligated to provide both tribute and military service to the king. Consequently, the imperial army was composed of a diverse range of forces, including:1

• Land Forces: These consisted of Persian and Median cavalry, archers, and sparabra (shield bearers), supplemented by the best troops recruited from the various subject peoples.1
• Naval Forces: The fleet was assembled by drawing ships and crews from the Greek states of Asia Minor, as well as from Phoenicia, Cyprus, and Egypt.1

By commanding these resources as the "king of kings," Xerxes was able to mobilize a vast, multi-ethnic military force to support his westward expansion.21